I don't think there is an automatically way to do your request. However, you can do this in some steps:
Step 1
Check the number of tables in both of your databases. Maybe new modules insert some new tables.
Step 2
Suppose one of databases as initial and other as secondary. If they have same database prefix, change the second one to something else and import it to the initial database. In this way you could test it easily just by renaming the prefixes and table names.
Step 3
Your contents are related to these tables:
categories
content
content_frontpage
content_rating
Take care "categories" table might be used by some extensions too, recheck it to ensure everything is OK.
Step 4
Your extension related tables are:
modules
modules_menu
extensions
template_styles
Step 5
There is one table that share with all contents and extension:
assets
Check and merge this table manually.
Also you can use compare tools to help you in this case:
Compare two MySQL databases